It is known in the art of fiber amplitude-modulated continuous wave (AMCW) laser radar (ladar) systems to launch an amplitude-modulated frequency-chirped (or linearly ramped frequency) optical signal toward a target, typically traveling partially along an optical fiber. The range of frequencies over which the amplitude is chirped is typically in the microwave range. The light reflects off the target, propagates back to the fiber, and is detected by an optical detector. Because the signal is frequency-chirped, the frequency of the transmitted (or reference) signal will be different from (typically ahead of) the frequency of the return (or reflected) signal at the time the return signal is detected. This frequency difference will be indicative of the time delay between the transmitted and reflected signals (knowing the chirp-rate of the transmitted signal). The time delay is related in a known way to the distance to the target. Such a system is described in the article: G. L. Abbas et al, "Ladar Fiber Optic Sensor System for Aircraft Applications," SPIE Vol. 1799 (1992) pgs. 120-124.
Also, as is known, the range or distance to the target is a function of the chirped bandwidth (i.e., the range of microwave frequencies over which the optical amplitude is modulated), the chirp duration time over which the frequency is ramped, and the speed of light. Accordingly, the error associated with the determination of range to the target is proportional to the square root of 1/B.sup.2 TP.sub.s ; where B is the chirp bandwidth, T is the chirp duration, and P.sub.s is the signal power returning to the detector, as is discussed in the aforementioned Abbas article.
Current systems use a microwave electronic drive circuit to modulate the optical amplitude of a laser diode over a microwave frequency range, 2-8 GHz, to provide the amplitude-modulated frequency-chirped transmitted signal. However, this requires expensive and complex electronics. Also, due to the frequency limitations of microwave electronic circuits, the chirp bandwidth is limited to about 10 GHz, thereby fixing the minimum range error at a corresponding value related thereto. Also, the power of a typical laser diode capable of providing such a source is about 1 mW, thereby also fixing the minimum range error to something proportional thereto.
Thus, it would be desirable to provide an amplitude-modulated frequency-chirped microwave source that can be adapted to such a ladar fiber optic sensor which allows for reduced range error over present microwave chirp sources and reduces the cost of presently-used electronic sources.